Micro-machined structures use the techniques of deposition, etchings, ion implantations, etc. developed in the field of electronic integrated circuits and then extended to other fields. The scale of the micro-machined elements can drop below, or indeed even greatly below, a micrometer. These structures are either purely electronic (integrated electronic circuits) or mixed, involving for example optical elements (image sensors, displays, optical modulators for example) or mechanical elements (accelerometers, pressure sensors, for example), or even chemical (chemical sensors) or biological (bio-sensors) elements.
These structures are manufactured collectively on plane support wafers which are usually made of silicon when the structures comprise electronic elements.
During this fabrication, it may be required to execute steps of very precise alignment between two structures, for example with a view to a precise gluing of an individual microchip (already cut) onto a wafer, or else with a view to gluing two wafers together, with elements of the first wafer placed very precisely opposite elements of the second wafer. Such is typically the case when it is desired to interconnect several microchips by gluing them on a wafer which carries interconnection conductors. In this case, the contacts of the microchip must be precisely aligned with corresponding contact tags of the wafer.
In general, the alignment of the microchip on the wafer or the alignment of the first wafer on the second wafer is measured by optical procedures, often in the infrared (for which silicon is transparent) because it may be necessary to measure an alignment through the thickness of the microchip or wafer. Use is made of optical alignment marks formed on the two items to be glued. These measurements make it possible either to put the items in place during fabrication or to check by measurement the degree of possible misalignment after fabrication.
Sometimes, alignment is facilitated by hollowing out deep trenches in the silicon at the location of the alignment marks, so that the light rays which allow the alignment to be checked pass more easily through the thickness of the microchip or wafer.
For destructive measurements of misalignment, it is possible to use electron microscopes and observe the structures cut (and therefore destroyed).
Consideration has also been given to performing mechanical alignment by nesting male and female patterns machined or photolithographed on the items to be placed opposite one another, but this does not allow measurements of alignment quality to be made.
Thought has also been given to aligning items by magnetic means, namely magnetic micro-domains deposited on each of the items and which mutually attract one another while the items are being put in place so as to favour well defined alignment. Here again, no measurement is possible.
Optical measurement procedures remain the most effective but suffer from drawbacks, in particular that of the insufficient transparency of the substrate when the alignment requires seeing through a substrate. If the substrate is made of silicon, one is forced to use infrared radiation in order for the radiation to pass through it, and the precision of alignment can scarcely drop below half the wavelength of the radiation used. Even with infrared radiation it is still necessary for the silicon not to be overly doped, the doping reducing its transparency, and it is necessary to prevent there being any metallic interconnection layers in the vicinity of the alignment marks. These marks take up room. Moreover, the layer of glue interposed between the microchip and the wafer or between two wafers must itself be transparent to infrared.